There has been a demand for wide-ranged variable speed control, reduction in electricity consumption, and improved maintainability in the field of compressor motors and fan motors in applications such as air conditioners, and in the field of drive motors for electric automobiles. To meet such demands, a sensor-less drive method is being used in a number of applications. The sensor-less drive method executes field-oriented control of a permanent magnet motor having a permanent magnet in its rotor with an inverter unit without providing a sensor such as hall IC for rotor positioning detection purposes.
For instance, fan motors used in exterior units of air conditioners experience abnormal rotation in which the motor becomes locked unrotatably by external force or by its own problems, and in some instances experience significant reduction in rotational speed by step-out, or by other reasons. A sensor-less drive method that does not employ a sensor for detecting rotor positioning is not capable of directly detecting such abnormalities in rotation. In the light of the above, in JP 2003-319698 A, angular frequency of the rotor is detected by converging the d-axis induced voltage estimate to zero when executing a position estimate operation to determine the occurrence of abnormal rotation based on the difference between the detected angular frequency and the angular frequency command value.
The above described patent document discloses means for determining rotational abnormalities during the position estimation operation. However, when starting the motor, forcible commutation by way of a conductive signal having a constant pattern is required until induced voltage reaches a substantial magnitude to allow position detection of the rotor. Since forcible commutation does not provide rotational position feedback, rotational abnormalities such as step-out is prone to occur, and such rotational abnormalities also introduces problems such as noise generation.